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1. First-principles study of the electronic structure, Z 2 invariant, and quantum oscillation in the kagome material CsV3Sb5

   https://doi.org/10.1063/5.0232167  

   Bhandari, Shalika R; Zeeshan, Mohd; Gusain, Vivek; Shrestha, Keshav; Rai, D P (December 2024, APL Quantum)

   NA (Ed.)

   This work presents a detailed study of the electronic structure, phonon dispersion, Z2 invariant calculation, and Fermi surface of the newly discovered kagome superconductor CsV3Sb5, using density functional theory. The phonon dispersion in the pristine state reveals two negative modes at the M and L points of the Brillouin zone, indicating lattice instability. CsV3Sb5 transitions into a structurally stable 2 × 2 × 1 charge density wave (CDW) phase, confirmed by positive phonon modes. The electronic band structure shows several Dirac points near the Fermi level, with a narrow gap opening due to spin–orbit coupling (SOC), although the effect of SOC on other bands is minimal. In the pristine phase, this material exhibits a quasi-2D cylindrical Fermi surface, which undergoes reconstruction in the CDW phase. We calculated quantum oscillation frequencies using Onsager’s relation, finding good agreement with experimental results in the CDW phase. To explore the topological properties of CsV3Sb5, we computed the Z2 invariant in both pristine and CDW phases, resulting in a value of (ν0; ν1ν2ν3) = (1; 000), suggesting the strong topological nature of this material. Our detailed analysis of phonon dispersion, electronic bands, Fermi surface mapping, and Z2 invariant provides insights into the topological properties, CDW order, and unconventional superconductivity in AV3Sb5 (A = K, Rb, and Cs). 

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2. Fermi surface reconstruction under pressure in the kagome metal ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$

   https://doi.org/10.1103/PhysRevB.110.205135  

   Phillips, Cole; Shtefiienko, Kyryl; Nguyen, Thinh; Capa_Salinas, Andrea N; Magar, Birendra A; Pokharel, Ganesh; Wilson, Stephen D; Graf, David E; Shrestha, Keshav (November 2024, Physical Review B)

   NA (Ed.)

   This work presents the evolution of the electronic properties of kagome superconductor CsV3Sb5 under pressure. The magnetoresistance under high fields of 43 T showed clear Shubnikov–de Haas (SdH) oscillations with multiple frequencies up to 2000 T. With the application of pressure, we observed a sudden change in SdH oscillations with the disappearance of the high-frequency signal near the critical pressure Pc1 ∼ 0.7 GPa. We argue that this change could be due to a reconstruction of the Fermi surface (FS) in CsV3Sb5. To interpret our experimental data, we computed the electronic band structures and FS of CsV3Sb5 using ab initio density functional theory. Our results indicate that both the electronic bands and FS of CsV3Sb5 are highly sensitive to external pressure. The deformation of FS pockets with increasing pressure qualitatively explains our experimental observations. The pressure-driven FS instability in CsV3Sb5 may induce changes in its electronic states, such as superconductivity, charge density wave, nontrivial topology, and more. Therefore, these results are invaluable for gaining insights into these electronic states in CsV3Sb5, as well as in other kagome materials. 

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3. Effect of Fermi surface topology change on the Kagome superconductor CeRu2 under pressure

   https://doi.org/10.1016/j.mtphys.2023.101322  

   Deng, Liangzi; Gooch, Melissa; Liu, Hongxiong; Salke, Nilesh P.; Bontke, Trevor; Shao, Sen; You, Jingyang; Schulze, Daniel J.; Kumar, Ravhi; Yin, Jia-Xin; et al (January 2024, Materials Today Physics)

   - (Ed.)

   The cubic Laves phase compound CeRu2 with a Kagome substructure of Ru has been investigated to understand myriad fascinating phenomena resulting from competition among its various physical and geometric features. Such phenomena include flat bands, van Hove singularities, Dirac cones, reentrant superconductivity, magnetism, the Fulde–Ferrell–Larkin–Ovchinnikov state, valence fluctuations, time-irreversible anisotropic s-state superconductivity, etc. Extensive studies have thus been carried out since 1958 when the highly unusual coexistence of superconductivity and ferromagnetism was proposed for the mixed compounds (Ce,Gd)Ru2. Activity has accelerated in recent years due to increasing interest in topological states in superconductors. However, there has been little investigation of the mutual influence of these fascinating states. Therefore, we systematically investigated the superconductivity and possible Fermi surface topological change in CeRu2 via magnetic, resistivity, and structural measurements under pressure up to ~168 GPa. An unusual phase diagram that suggests an intriguing interplay between the compound’s superconducting order and Fermi surface topological order has been constructed. A resurgence in its superconducting transition temperature was observed above 28 GPa. Our experiments have identified a structural transition above 76 GPa and a few tantalizing phase transitions driven by high pressure. Our high-pressure results further suggest that superconductivity and Fermi surface topology in CeRu2 are strongly intertwined, 

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4. Creating superconductivity in WB2 through pressure-induced metastable planar defects

   https://doi.org/10.1038/s41467-022-35191-8  

   Lim, J.; Hire, A. C.; Quan, Y.; Kim, J. S.; Xie, S. R.; Sinha, S.; Kumar, R. S.; Popov, D.; Park, C.; Hemley, R. J.; et al (December 2022, Nature Communications)

   Abstract High-pressure electrical resistivity measurements reveal that the mechanical deformation of ultra-hard WB 2 during compression induces superconductivity above 50 GPa with a maximum superconducting critical temperature, T c of 17 K at 91 GPa. Upon further compression up to 187 GPa, the T c gradually decreases. Theoretical calculations show that electron-phonon mediated superconductivity originates from the formation of metastable stacking faults and twin boundaries that exhibit a local structure resembling MgB 2 (hP3, space group 191, prototype AlB 2 ). Synchrotron x-ray diffraction measurements up to 145 GPa show that the ambient pressure hP12 structure (space group 194, prototype WB 2 ) continues to persist to this pressure, consistent with the formation of the planar defects above 50 GPa. The abrupt appearance of superconductivity under pressure does not coincide with a structural transition but instead with the formation and percolation of mechanically-induced stacking faults and twin boundaries. The results identify an alternate route for designing superconducting materials. 

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5. Creating superconductivity in WB2 through pressure-induced metastable planar defects

   Lim, J.; A. C. Hire; Y. Quan; J. S. Kim; S. R. Xie; S. Sinha; R. S. Kumar; D. Popov; C. Park; R. J. Hemley; et al (January 2022, Nature communications)

   - (Ed.)

   High-pressure electrical resistivity measurements reveal that the mechanical deformation of ultra-hard WB2 during compression induces superconductivity above 50 GPa with a maximum super-conducting critical temperature, Tc of 17 K at 91 GPa. Upon further compression up to 187 GPa, the Tc gradually decreases. Theoretical calculations show that electron-phonon mediated super-conductivity originates from the formation of metastable stacking faults and twin boundaries that exhibit a local structure resembling MgB2 (hP3, space group 191, prototype AlB2). Synchrotron x-ray diffraction measurements up to 145 GPa show that the ambient pressure hP12 structure (space group 194, prototype WB2) continues to persist to this pressure, consistent with the formation of the planar defects above 50 GPa. The abrupt appearance of superconductivity under pressure does not coincide with a structural transition but instead with the formation and percolation of mechanically-induced stacking faults and twin boundaries. The results identify an alternate route for designing superconducting materials. 

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